The present invention relates to an engine braking control in an automatic transmission.
Automatic transmissions effect a shift between gears by change-over in power delivery path after selective actuation of one or ones of various kinds of friction elements. It is the commonly employed practice to provide a one-way clutch which serves as a reaction member against a rotary member to establish a power delivery path.
With this one-way clutch, if the rotary member is subject to reverse torque, the one-way clutch is released to interrupt the transmission of this reverse torque. However, since the one-way clutch interrupts the transmission of reverse torque, no effective engine braking results.
Thus, a known automatic transmission is provided with a friction element which is arranged in parallel to a one-way clutch such that the friction element is activated to effect engine braking when a driver places a manual selector valve at a predetermined range position.
The engine braking control of this kind is employed for example in a THM-700 type transmission manufactured by General Motors Corporation. In this transmission, when a dirver places a manual selector valve to "III" or "II" or "I" drive range for effective engine braking in the speed range selected, a friction element (i.e., an overrunning clutch) is activated by a range pressure such as "III" range pressure or "II" range pressure or "I" range pressure, thus effecting engine braking during running in the corresponding drive range.
According to this known transmission, the friction element provided for effecting engine braking is kept activated since the driver placed the manual selector valve at one of the drive ranges. Thus, under this condition, if the driver manipulates the accelerator pedal such that depression degree frequently, the transmission of reverse torque is not interrupted so that occurrence of shocks, noises and vibrations is caused by the transmission of reverse torque. Since the transmission of reverse torque is not interrupted, if such reverse torque results for shifting operation, so-called shift shocks due to the reverse torque will take place.
An object of the present invention is to provide an engine braking control in an automatic transmission which is free from the above mentioned problems.